cutlass stalling

Hello - I have had an intermittent problem with a 1996 Cutlass Supreme stalling while in traffic. It does not overheat, just stalls and restarts in 15 minutes. After years of searching, finally one mechanic said it was the head gasket leaking antifreeze. So that was fixed 9 months ago. Now my son claims that it stalled twice in traffic last night, he pulled off and let it rest a few minutes, then it restarted. Any ideas?